Hi Thomas,
If you need reentrance it's also possible to turn the far address and
a bit of the buffer address on the stack into a piece of code
(Addr_MSB = 0xAF, MSB_buf = 0x81) and call it using the Y register.
Documenting such a solution would be "interesting" (an example for
adding "missing" addressing modes that way is here
<https://github.com/TG9541/stm8ef/blob/4a5053236605787c23c322ea39a51a1072f3db35/forth.asm#L892>,
and a bit about how SDCC represents parameters on the stack is here
<https://github.com/TG9541/stm8ef/wiki/STM8S-Programming#combining-c-and-assembly>).
That's an interesting idea. So, if I am understanding correctly, I could
avoid the use of static temporary variables (which is what is preventing
re-entrancy) by instead converting the stack arguments of my function to
code, and directly executing that?
Let's see: to turn the 32-bit 'addr' variable into a "LDF ($1234, X), A"
instruction, we make use of the unused high byte and set it to 0xA7, the
opcode for that LDF operation, and the following three bytes remain as
the source address: 0x00027F80 => 0xA7027F80. Okay, cool.
But I don't see how the 'buf' variable could be turned into an "LD A,
($1234, X)" instruction, because that needs 3 bytes, and the variable is
only 2!
Also, I need to *precede* the LDF with an LD instruction (remember, the
purpose of my function is to write the contents of buf to addr, not the
opposite), but if my function has the signature of
"flash_write_block(const uint32_t addr, const void *buf)", the arguments
on the stack will be ordered as addr then buf, which is the opposite of
what's needed.
And where would a return instruction get put? I am assuming this
stack-as-code stuff would get invoked like this:
ldw y, sp ; copy stack pointer value
addw y, #0x04 ; offset to the first function arg
call (y) ; make the jump
Therefore necessitating a RET instruction to be a final part of the code
on the stack. How?
Regards,
Basil
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